This invention relates to threaded fastener assemblies and, more particularly, to assemblies in which the fastener is captivated within the workpiece.
A common goal among manufacturers is to increase productivity by reducing the production time of an assembly process. In an effort to achieve this goal, captivated fastener assemblies are often employed. Such captivated fastener assemblies utilize fasteners which can be pre-attached to a workpiece that later is fastened to a support structure during the assembly process. Because attempts to find lost or misplaced screws are both inconvenient and time consuming, captivated fastener assemblies are used to help reduce the production time of an assembly process and wasted worker effort that is caused by searching for these lost or misplaced screws. In addition, captivated fastener assemblies can ease workpiece installation processes by holding the fastener within the workpiece in proper position to allow a worker to expediently secure the workpiece to the support structure.
Many captivated fastener assemblies have previously been designed in which the fastener is retained within the washer to prevent separation. FIGS. 1 and 2 illustrate a known captivated fastener assembly which is used to secure a valve cover (not shown) to an engine cylinder head (not shown). The assembly 10 generally includes three pieces: fastener 12, sleeve 14 and grommet 16. It is necessary that the fastener 12 be able to slide axially within the bore of the sleeve 14. On the other hand, it is also important that the assembly 10 remain assembled and not fall apart during shipment or when the cover is mounted to the engine cylinder head. Thus, there is a requirement that the fastener 12 be captivated within the workpiece. A common commercially used design for accomplishing these goals employs a protuberance, such as enlarged ring 18, formed on the fastener shank which cooperates with a reduced diameter section 20 formed in the sleeve 14. The outer diameter of the ring 18 and inner diameter defined by the sleeve section 20 are such that the fastener can be pushed through the sleeve so that the ring 18 falls below section 20 yet there still remains a sufficient interference between the ring 18 and section 20 to prevent the ring (and thus the fastener) from again passing upwardly past section 20 which otherwise would permit separation of the fastener 12 from the sleeve 14.
The captivated fastener assembly disclosed in U.S. Pat. No. 4,732,519 employs a similar type of the above-mentioned arrangement. The captivated fastener assembly disclosed in this patent includes a fastener, washer, and grommet. A protuberance in the form of a set of wings projects from the shank of the fastener and cooperates with a washer sleeve. The washer sleeve includes an inwardly projecting annular bead portion pre-formed at the top of the sleeve. The wings of the fastener shank are designed to provide an outer diameter that is larger than the inner diameter of the annular bead portion of the sleeve. The fastener is captivated within the washer by forcing the wings of the fastener past the annular bead portion to provide an interference fit, thereby preventing the upward removal of the fastener.
In general, the afore-mentioned captivated fastener assemblies require special steps for forming both the fastener and the washer. For example, the fastener is typically formed by a special thread rolling process. A forging process is normally needed to form the protuberance.
It is very important that tight tolerances be kept during the forming processes. If the difference between the relative diameters is too large, then it could be very difficult or impossible to insert the fastener into the washer sleeve. In contrast, if the diameter of the protuberance of the fastener is too small, then the assembly could become inadvertently separated.
Furthermore, since the captivation process in these types of prior art examples requires that the fastener be forcibly inserted into an aperture, the fastener may axially elongate the grains of the material at the sleeve aperture surface. This action generates an undesirable axially oriented tensile stress gradient in the surface grains. These tensile stress gradients tend to decrease the ductility and fatigue life of the material at the sleeve aperture surface. This method may also cause deformation of portions of the fastener, such as the threads, thereby limiting its fastening effectiveness.
Therefore, it would be desirable to provide a captivated fastener assembly that overcomes one or more of these problems.
Accordingly, it is an object of the present invention to provide an assembly in which the fastener is captivated within the workpiece without the need for a distinct outward protuberance on the fastener shank. Thus, a conventional threaded fastener can be used with a workpiece having a deformable sleeve, yet the same goal obtained. The captivation of the fastener is obtained by employing the deformable workpiece sleeve which, before deformation, accepts the shank of the fastener and slides freely along the shank. After the workpiece is slid over the fastener, a plurality of deformed segments are created by applying an inward compressive force to the outside of portions of the sleeve. The inner diameter provided by the deformed segments remains greater than the diameter of the unthreaded portion of the fastener such that a free-flowing fit between the deformed segments and the unthreaded portion of the fastener is provided. Therefore, the fastener is able to move axially relative to the workpiece. However, the inner diameter provided by the deformed segments is less than the outer diameter of the threaded portion of the fastener. Thus, the outer tips of the threads engage the deformed segments if the fastener moves too far upwardly, thereby preventing the fastener from being removed from the workpiece.